For over a century, our sense of smell has been explained with the "lock and key" hypothesis, which holds that each odor molecule has a particular shape that allows it to fit into particular smell receptors in the nose. But now a controversial study involving fruit flies suggests that hypothesis might miss the truth entirely - the secret, they say, is all in the vibrations.

The lock and key gets its hypothesis because all the odor molecules have a matching receptor, just like a key will fit a particular lock. The problem is that there are only a few hundred smell receptors in the nose, and we know that humans can detect tens of thousands of different odors. Besides, if shape is so important, you would think it had more of a relationship with what molecules smelled like. That often isn't the case - for instance, the vodka and rotten eggs odor molecules are practically identical, but they obviously smell nothing alike. (Well...that might depend on the quality of the vodka.)

But we also know that molecules can vibrate, and even molecules that are nearly the same in atomic structure can have wildly different vibration patterns. That might be the missing factor that allows such a wide variety of smells to be interpreted by a relatively small amount of receptors. MIT researchers decided to test the theory:

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"The researchers focused on Drosophila, or fruit flies, which don't bring the same subjective odor experiences to the table that a human might. They placed the flies in a maze with two arms, into which they pumped nearly identical odor molecules. Odorants such as acetophenone and deuterated acetophenone, for example, have the same molecular structure; one is just built from a slightly heavier hydrogen atom, known as deuterium. Despite these miniscule variations, the flies showed a consistent preference for one arm of the maze over the other...suggesting that the flies could tell the difference between the odorants."

The only real difference between the two odor molecules that the flies encountered was in the vibrational patterns, which suggests that was somehow what the flies detected in order to distinguish between them. It's not clinching proof by any means, but it's good evidence that flies do indeed use vibrations as part of their smell apparatus. But that doesn't necessarily prove the same is true for humans, as Rockefeller University neuroscientist Leslie Vosshall points out:

"I think this paper nicely demonstrates that flies can tell deuterated and non-deuterated odors apart. I do not think that this result on its own either confirms or refutes the vibration theory."

The next step will be to try to replicate these sorts of results in mammals' smell receptors, efforts which in the past have met with failure. The ultimate truth may well lie somewhere in the middle - vibration might play a role in the human sense of smell, but the lock and key hypothesis might still have a part to play as well.